I just apply Future to the general area where the decal will be.  Whatever you use to top-coat the decal (gloss, semi-gloss, flat) will be much thicker than the Future and will give a uniform finish.  The decal is more than 5 times thicker than the Future.

See photos of models at :
https://harley-trains.smugmug.com/PFEModels

Dick Harley
Laguna Beach,  CA